You are aware encrypting datas is not compressing datas ?
Because most (i won't say all, but i think that) encryption higher the datas size, and any bit change in datas will trigger re-encryption of datas.

Using encryption with SSD is then an action that should only be made by crazy users, or rich ones.

I'm sorry i don't have your answer for your question, but if i could let you save some bucks.

Thanks for your concern. As far as I know, cryptsetup has TRIM support since V1.4.
Bad thing on using TRIM on encrypted SSD is about possible footprints (discarded sectors) which might lead to decreased security [1]

This should be sufficient - if pam_mount detects the "discard" option, it should add --allow-discards to cryptsetup call.
PS: this is for really simple use case of having sda3 with LUKS/AES-XTS/Btrfs partition containing my user folder. I am the only user of this laptop, so this is the most appropriate scenario for me.
PS2: The patch was commited in January 2012 (after this thread was started), but I am not sure since which version of pam_mount the patch is incorporated.